I Have Been Taken Synthroid And Now Levothyroxin Since 1994

I have been taken synthroid and now levothyroxin since 1994 and in a year I have had to increase my dose 3 times. I am now taking200 mcg. Because it was so hard to get my levels right my dr. had an ultrasound done and they found my thyroid was shrinking and I hardly have any thyroid left. what causes it to shrink and is that normal. also what do you think of that Armour thyroid medicine? my last question for now is should I look for vitamins that do not have iron in it , or eat cereals that have high iron, or liver.